[FEATURE](libnpmversion) Preserve build metadata, and perhaps provide a way to set it

What / Why

Preserve semver build metadata if specified, rather than stripping it off.

It's perfectly safe to publish a package with build metadata in the package.json (since we strip it at every other place versions are consumed or analyzed).

Current Behavior

npm version 1.0.0+build.1

Writes "version": "1.0.0" to package.json.

How

• If a version is specified, then do not strip build metadata from it.
• Instead of passing the version through semver.valid() (which strips build metadata), use semver.parse() and stringify it in such a way that build metadata is preserved. (May benefit from an upgrade to node-semver to make a "blessed" way to do this, rather than sniffing the object and manually appending the string.)
• Include build metadata in git tag/commit as well as package.json file.
• (maybe?) add a buildMetadata field to the options this module accepts, so you could do something like npm version patch --build-metadata=foo.bar.1.3.0xCAFEBAD to append build metadata while programmatically bumping the version.

References

• https://github.com/npm/cli/issues/1479

Start in libnpmversion and trace the handling of a specified version through semver.valid() and semver.parse(). Check how package.json versions and git tags or commits are written. Done means preserving build metadata in package.json and the related git tag/commit, with the optional buildMetadata setting considered separately.